Only 1 of my text letters turns black
I was stumped by this issue as well. I was able to fix it by selecting the text, going under the "Object Data" property, and making sure the "Size" (under the Font menu) isn't set too low. In my case, anything less than .01 caused the problem.
If you set it too low, some letters go black. Raise it back up, they go back to normal. She falls down a well, eyes go crossed. She gets kicked by a mule, they go back to normal. I don't know.

If you need to get the text down past the point of the darkness setting in, just scale it instead of changing the size value.